1. Field of the Invention
The present invention relates to novel polymerizable solid aliphatic polyurethanes containing olefinically unsaturated double bonds and based on linear diisocyanates. The present invention also relates to the use of the novel aliphatic polyurethanes as novel powder coating materials or for preparing novel powder coating materials. The present invention relates not least to the use of the novel powder coating materials for producing coatings on primed or unprimed substrates.
2. Description of the Background
German patent application DE-A-24 36 186 discloses a polyurethane prepared from tolylene diisocyanate, an aromatic nonlinear diisocyanate, 2-hydroxyethyl methacrylate, and trimethylolpropane in a molar ratio of 3:3:1. The polyurethane has a melting point of approximately 65xc2x0 C. and a polymerizable double bond content of 2.9 double bonds per 1000 molecular weight. It can be used per se as a powder coating material curable with actinic radiation. Owing to the presence of aromatic structures, the coatings produced from it are not stable to weathering but instead tend to yellow under the effect of sunlight.
European patent application EP-A-0 410 242 discloses polyurethanes containing (meth)acryloyl groups in an amount corresponding to from 3 to 10% by weight, based on the polyurethane, of xe2x95x90Cxe2x95x90Cxe2x95x90 (molecular weight 24). These known polyurethanes have unspecified melting points or melting ranges in the temperature range from 50 to 180xc2x0 C. They are prepared using isophorone diisocyanate, 4,4xe2x80x2-diisocyanatodicyclohexylmethane, 4,4xe2x80x2-diisocyanatodiphenylmethane, its technical-grade mixtures with 2,4-diisocyanatodiphenylmethane, and, if desired, the higher homologs of these diisocyanates, 2,4-diisocyanatotoluene and its technical-grade mixtures with 2,6-diisocyanatotoluene (tolylene diisocyanate), and also biuret-, isocyanurate- or urethane-modified polyisocyanates based on these simple polyisocyanates. As regards the polyurethanes based on aromatic polyisocyanates, the comments made above apply. Additionally it is difficult to use these polyisocyanates as a basis for preparing polyurethanes having a particularly narrow melting range, let alone a defined melting point. In particular, the use of polyisocyanates having an average functionality  greater than 2 leads to polyurethanes having an undesirably broad molecular weight distribution, making them of only limited usefulness in one-component powder coating materials.
It is an object of the present invention to find novel polymerizable solid polyurethanes, containing olefinically unsaturated double bonds, which no longer have the disadvantages of the prior art but instead possess a very narrow melting range within the temperature range 40 to 200xc2x0 C. and can be used as powder coating materials or for preparing powder coating materials which give weathering-stable and nonyellowing coatings.
Found accordingly has been the novel polymerizable solid aliphatic polyurethane containing olefinically unsaturated double bonds and having a very narrow melting range within the temperature range from 40 to 200xc2x0 C., which is preparable from
A) at least one linear aliphatic diisocyanate,
B) at least one aliphatic compound containing at least two isocyanate-reactive functional groups and/or water, and
C) at least one olefinically unsaturated compound containing an isocyanate-reactive functional group.